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Rotor wake turbulence: An experimental study of a wind turbine wake

Eriksen, Pål Egil
Doctoral thesis
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URI
http://hdl.handle.net/11250/2382264
Date
2016
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  • Institutt for energi og prosessteknikk [2630]
Abstract
An experimental investigation of the first five diameters of the wake of a 0.9

meter in diameter model wind turbine with three blades has been undertaken.

The blades are twisted and tapered and uses a NREL S826 profile

along the full length of the blade. The test environment is a closed loop

wind tunnel with a cross-section of 1.8 by 2.7 meters, which produces a

uniform flow with 0.24% turbulence intensity. All measurements are performed

at the turbines design condition, at which R = 6 and Retip 105.

This measurement campaign is an extension of the experimental work undertaken

by the author for the 2011 blindtest workshop arranged at NTNU

by NOWITECH and NORCOWE where the numerical community was invited

to predict the development of the wake. The results of the blindtest

were reported by Krogstad and Eriksen (2013).

High-speed measurements are obtained with a four wire hot-wire probe operated

at constant temperature which can resolve all three components of

the velocity vector. An existing data reduction scheme proposed by Maciel

and Gleyzes (2000) has been modified to work over the wide range of velocities

and flow angles encountered in a wind turbine wake. In addition to

the velocity vector, the rotor position was measured simultaneously. This

allowed for conditional averaging of the acquired data, which made it possible

to reveal periodic coherent structures in the flow. The investigation

has also looked at conventional time averaged statistics and frequency and

wave-number spectra. The results from these different methods of analysis

have been used to estimate terms in the energy budgets of the mean, periodic

and turbulent motions in the flow.

The analysis reveals how the wake develops from a flow dominated by periodic coherent structures to one where purely turbulent motions governs

production of turbulent kinetic energy and transport of momentum into the

wake. The coherent motions do initially contain a significant portion of the

time averaged turbulent kinetic energy near the edge of the wake and are

found to be important in both production and radial transport of turbulent

kinetic energy. The vortices do naturally dominate the spectral content of

the initial wake. Investigations of wave-number spectra has revealed that

while the energy containing range gradually moves towards larger scales

as the periodic coherent structures decay, a significant inertial sub-range

emerges, parts of which can be described as isotropic. After the collapse of

the tip vortex system the mean dissipation and production has been found

to balance and the evolution of the turbulent kinetic energy level is governed

by radial diffusion.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2016:34

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